1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control, and more particularly to an improved structure of a mechanical seal which keeps a reference gas chamber and a gas chamber airtight in a gas sensor.
2. Background Art
Gas sensors are know which are fabricated by inserting a sensor element into an insulation porcelain, mounting the insulation porcelain in a housing, installing a gas cover and an air cover on a front end and a base end of the housing, respectively, and sealing a gap between the insulation porcelain and the housing and a gap between the sensor element and the insulation porcelain hermetically. These seals define a measured gas chamber and an air chamber within the gas sensor in an airtight fashion.
The sensor element has a measuring electrode exposed to a gas to be measured and a reference electrode exposed to a reference gas or air and provides a signal in the form of an ion current flowing through the measuring and reference electrodes or a potential difference between the measuring and reference electrodes to determine the concentration of the gas. The leakage of the gas from the measured gas chamber to the air chamber or vice versa will, thus, result in a decrease in accuracy of measuring the concentration of the gas. In order to avoid this problem, typical gas sensors pack glass powder in the insulation porcelain and melt and cool it to produce a high density solid sealing member within the gap between the sensor element and the insulation porcelain.
Typically, the sensor element and the insulation porcelain are made from zirconia and alumina, respectively. These materials are different in thermal expansion coefficient, thus causing the sensor element and the insulation porcelain to expand or shrink greatly at different rates, especially in a case where the gas sensor is installed in an extreme environment such as an exhaust system of an automotive engine in which the gas sensor experiences a great temperature change from a higher temperature level of exhaust gasses to a lower temperature level after a stop of the engine, which may result in formation of cracks in the sealing member in the insulation porcelain, thus leading to a decrease in degree of airtightness between the sensor element and the insulation porcelain.
In order to avoid such a problem, Japanese Patent First Publication No. 3-167461 (equivalent to U.S. Pat. No. 5,228,975) teaches limiting a difference in thermal expansion between a glass seal and a housing to a specific range. It is, however, difficult for the glass seal to absorb the expansion and shrinkage thereof completely. In order to alleviate such a problem, additional parts such as a spacer and a ceramic insulator are needed, which results in an increase in manufacturing costs. The structure of the above publication has also a problem that the expansion and shrinkage of the glass seal may cause the surface of the sensor element to be peeled off and the sensor element to be broken.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of a gas sensor which provides a mechanical seal required to keep a reference gas chamber and a gas chamber in the gas sensor airtight in an environment such as an automotive exhaust system subjected to a great temperature change.
According to the first aspect of the invention, there is provided a gas sensor which features a mechanical seal and which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control. The gas sensor comprises: (a) a hollow housing having a first and a second end portion; (b) a sensor element having a length which includes a first and a second portion; (c) a hollow insulating member disposed in the housing, retaining the sensor element therein; (d) a first cover installed on the first end portion of the housing to define a first chamber in which the first portion of the sensor element is exposed to a reference gas; (e) a second cover installed on the second end portion of the housing to define a second chamber in which the second portion of the sensor element is exposed to a gas to be measured; and (f) a glass sealing member disposed between an inner wall of the hollow insulating member and an outer wall of the sensor element to establish a hermetical seal between the first and second chamber. Differences in thermal expansion between the glass sealing member and the sensor element and between the sealing member and the insulation member are within a range of xc2x13xc3x9710xe2x88x926/xc2x0 C.
In the preferred mode of the invention, a composition of the glass sealing member contains, as expressed by conversion to oxide, the following components:
21.0xc2x15% by weight of B2O3,
34.6xc2x15% by weight of ZnO,
12.6xc2x15% by weight of SiO2,
4.9xc2x13% by weight of Al2O3,
14.2xc2x15% by weight of BaO, and
12.7xc2x15% by weight of MgO.
Note that the conversion to oxides is accomplished, for example, by separating the glass sealing member into metallic elements and typical elements in any known manner and oxidize them under high temperatures.
The glass sealing member may be made by melting glass powder under high temperatures and solidifying it within the insulating member. The use of material of the sealing member containing B2O3 and ZnO in the above weight percent range provides the differences in thermal expansion between the glass sealing member and the sensor element and between the glass sealing member and the insulation member which are within the range of xc2x13xc3x9710xe2x88x926/xc2x0 C.
When the quantity of ZnO is less than the above weight percent range, it will degrade the crystallization of the glass sealing member, thus resulting in an increase in adverse effect of mechanical properties of non-crystallized glass components contained in the glass sealing member. This requires use of the gas sensor in a lower temperature environmental condition. Alternatively, when the quantity of ZnO is greater than the above weight percent range, it will promote the crystallization of the glass sealing member, thus resulting in a decrease in amount of the non-crystallized glass components. This decreases the adhesion of the glass sealing member to the sensor element and the insulating member made of alumina, thereby causing the degree of airtightness between the first and second chambers to be decreased.
When the quantity of BaO and MgO are within the above weight percent ranges, it enables desired ones of deposited crystals having coefficients of linear thermal expansion different from each other greatly to be balanced with each other, which allows the coefficient of linear thermal expansion of the glass sealing member to be brought close to that of the insulating member made of alumina.
When the quantity of Al2O3 is less than the above weight percent range, the crystallizing temperature of the glass sealing member will be close to the softening temperature and the glass transition point, so that the material of the glass sealing member is crystallized shortly after it is softened. This results in an increase in viscosity of the material of the glass sealing member, which causes the material to be solidified before filling up a gap between the sensor element and the insulating member, so that some leakage paths will be formed.
Alternatively, when the quantity of Al2O3 is greater than the above weight percent range, it arrests the crystallization of the material of the glass sealing member, thus resulting in an increase in adverse effect of mechanical properties of non-crystallized glass components contained in the glass sealing member. This requires use of the gas sensor in a lower temperature environmental condition.
The composition of the sealing member may alternatively contain, as expressed by conversion to oxide, the following groups of components:
(a) 21.0xc2x15% by weight of B2O3,
xe2x80x8332.0xc2x15% by weight of ZnO,
xe2x80x8319.0xc2x15% by weight of SiO2,
xe2x80x8312.0xc2x15% by weight of BaO, and
xe2x80x8317.0xc2x15% by weight of MgO.
(b) 26.0xc2x13% by weight of B2O3,
xe2x80x8345.0xc2x15% by weight of ZnO,
xe2x80x8314.0xc2x13% by weight of SiO2,
xe2x80x837.5xc2x13% by weight of BaO, and
xe2x80x837.5xc2x13% by weight of MgO.
(c) 24.0xc2x15% by weight of B2O3,
xe2x80x8357.5xc2x18% by weight of ZnO,
xe2x80x8311.0xc2x15% by weight of SiO2, and
xe2x80x837.5xc2x15% by weight of BaO.
(d) 22.6xc2x15% by weight of B2O3,
xe2x80x8334.5xc2x18% by weight of ZnO,
xe2x80x8312.8xc2x15% by weight of SiO2,
xe2x80x8311.5xc2x15% by weight of BaO, and
xe2x80x8318.0xc2x15% by weight of MgO.
(e) 19.0xc2x15% by weight of B2O3,
xe2x80x8330.4xc2x18% by weight of ZnO,
xe2x80x8316.0xc2x15% by weight of SiO2,
xe2x80x835.0xc2x13% by weight of Al2O3,
xe2x80x8320.0xc2x15% by weight of BaO, and
xe2x80x839.6xc2x15% by weight of CaO.
The differences in thermal expansion between the sealing member and the sensor element and between the sealing member and the insulation member is preferably within a range of xc2x12xc3x9710xe2x88x926/xc2x0 C.
The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.
In the drawings:
FIG. 1 is a longitudinal sectional view which shows a gas sensor according to the first embodiment of the invention; and
FIG. 2 is a graph which shows a relation between the coefficient of thermal expansion of a glass sealing member and the strength of an insulation porcelain and a sensor element.